# Planck length and Planck time

• B
• roineust
Is a single occurrence of light refraction in water, considered mathematically an addition of a dimension?

Is a single occurrence of light refraction in water, considered mathematically an addition of a dimension?
Whatever you mean to say here, it's coming across as nonsense. Try to formulate your question more clearly. If it's a new topic, start a new thread.

Does a single occurrence of light refraction in water, considered mathematically an addition of a dimension?
"Dimension" in this context is the physical dimensions of length ##L##, mass ##M## and time ##T##. For example, velocity has dimensions of ##LT^{-1}##,; force has dimensions of ##MLT^{-2}## and energy has dimensions of ##ML^2T^{-2}##.

This is not to be confused with spatial and time dimensions.

Something like ##\frac v c##, or ##\frac {m_1}{m_2}## which appears in a lot of mechanics problems, is dimensionless. This means also that these quantities are independent of the units. If the velocity is half the speed of light, then ##\frac v c = \frac 1 2## regardless of the units.

See:

https://en.wikipedia.org/wiki/Dimensional_analysis

roineust
Maybe we could remove the arbitraryness of the original question regarding units by assuming a base measure of length as 1 Lightsecond = 299796 km = 1 Flash [f]. By using this base, the Planck-Length would become 0.53*10^-43 f. So why is it only roughly half the length that light could cover in 1 Planck-Time (1*10^-43 s)?
If you define your unit of length to only rely on your unit of time (which you can do since there is such a well defined, prominent speed...), then it becomes irrelevant what you mean by "1 Second" as well, the ratio still is roughly 2 Planck-Length = 1 Planck-Time. It might be "irrelevant" to ask why - but then, why's that?

So why is it only roughly half the length that light could cover in 1 Planck-Time (1*10^-43 s)?
It isn't - your value for the Planck time is off by a factor of roughly two. The Planck time is 5.39×10-44s, which is consistent with your Planck length in light seconds - as it must be by definition.

"The Planck time is the time it would take a photon traveling at the speed of light to across a distance equal to the Planck length. " Is the, very sensible, answer to the original question, then. If I had just googled "planck time length" first...this was just such a "1st-post-idiocity" from me, it really made me laugh (and still smile as I type this, in a slightly embarrassed kind of way). Thank you Ibix.
"No. It just means that seconds are bigger than meters."
This just has to be the coolest answer, ever.

Last edited: